1. Field of the Invention
This invention relates in general to an adaptive equalizer, and more particularly to an analog adaptive equalizer that provides convergence of an error signal by, decoupling the error canceller, the automatic gain control and the filter.
2. Description of Related Art
"Communication" is the exchange of thoughts, opinions, ideas, and information. It is the means to socialize, do business, educate, and entertain. Communication can take many forms, such as spoken words, written letters, or symbols. Although face to face communication is often desirable, it is often not possible due to geographical distance, time constraints, and an ever-increasing need for a high volume of information in today's society. It is for this reason that information, or data, is sent over communications "channels," via "signals."
A communications channel is a single path for transmitting an electrical signal, such as a twisted wire-pair cable, or a fiber optic line. A signal is a physical representation of data, such as the electrical pulses which are used to correspond to digital logic levels.
Signals are sent, or transmitted, in a tremendous variety of forms. For example, signals are used to send voice information over a telephone line; modems use signals to transmit data between computers; signals are constantly sent between the CPU and disk storage device in a personal computer; and signals representing images and sound are transmitted from a television camera on-site, to the television in a viewer's living room that could be thousands of miles away.
Signal distortion or degradation is a significant problem in the field of communications. Any real communications channel has transmission deficiencies, including various kinds of noise and interference, which distort the signal. For example, static noise (caused by natural electric disturbances in the atmosphere) and thermal noise (caused by the random motion of electrons in the channel) are present to some extent in any communications channel. Intersymbol interference (degradation caused by imperfect channels) can also be a major problem. In short, there are many reasons why a signal that is sent may be unrecognizable when it is received.
Thus, transmission deficiencies must be corrected so that the signal received is the same as the one that was sent, and valuable information is not lost. This correction can be accomplished by the signal receiver, through a process known as equalization.
Equalization is the process of correcting a channel for its transmission deficiencies, by introducing networks which compensate for attenuation and time delay problems in the signal. A properly equalized communications channel will significantly increase the likelihood of obtaining an accurate signal (i.e., the signal that was sent) at the receiving end of a communications network. An "equalizer" is a device used to accomplish equalization.
A filter can be used as an equalizer. Further, a filter may have a means of monitoring its own frequency response characteristics and a means of varying its own parameters by closed loop action, in order to attain optimal equalization. Such a self-adjusting filter is called an "adaptive filter," and it can be used in a channel receiver to attain "adaptive equalization." The parameters of an adaptive filter are typically adjusted by sampling the filter output at a predetermined rate, and sending this sampled output to some filter control means, which adjusts filter parameters accordingly via closed loop feedback.
Commonly, a feed forward equalizer having weighted summed delayed versions of an input signal are used to derive an error signal for adaptively equalizing the input signal, e.g., remedying intersymbol interference. However, whenever such a design is implemented in an analog form, the adaptive equalizer becomes large and therefore difficult to implement. Other analog equalizers have not provided the adaptive equalization needed for accurate data communication.
It can be seen then that there is a need for an analog equalizer that is easily implemented and which is truly adaptive.
It can also be seen that there is a need for an analog adaptive equalizer that provides convergence of an error signal to provide truly adaptive error correction.